The present invention relates to a method of and apparatus for manipulating line weight in an image representation, and more particularly to a method of and apparatus for spreading or choking contour lines in slope mapping, wherein during exposure, light sensitive film is orbited parallel to an image-bearing positive or negative.
A slope map, defined as one which depicts bands or regions of terrain having a slope greater than or equal to a defined slope, is commonly used in studies related to land use. Typically, up to five or six such bands in different colors are displayed on the slope map representing, for example, terrain slopes ranging from 0 to 5%, from 5 to 15%, from 15 to 30%, from 30 to 50%, from 50 to 70%, and greater than 70%. The map is prepared at a predetermined scale, e.g., 1:24,000.
The slope map is generally prepared from a contour map negative containing contour lines which represent constant altitude regions, with adjacent contour lines representing a predetermined altitude differential, e.g., 10 feet (the contour lines are closer together in steep terrain than they are in relatively flat terrain). In order to form slope maps, the line thickness on a contour map negative is increased by an amount which depends upon map scale, contour interval and desired slope bands. As the contour lines are thickened, contour lines that are spaced close to one another coalesce to form a slope band representative of a region having a slope of at least a preselected value that depends on resultant line thickness. For example, where a relatively high slope band (representing only steep terrain) is required, e.g., 70% and greater, the contour lines are thickened by only a relatively small amount, and only contour lines that are very close to one another tend to coalesce. On the other hand where a relatively low slope band (representing relatively flat terrain as well as steep terrain) is required, e.g., 15% and greater, the contour lines are thickened by a substantial amount. In the latter case, even contour lines that are spaced apart by relatively large distances tend to coalesce.
The process of making a slope map requires first that a second contour map negative be prepared from the original negative with the contour lines thickened by a predetermined amount to define the required slope band. Then, with the contour lines thickened and the slope bands defined, a positive is made from the second negative. During production of the positive, contour lines outside of the slope band are made to undergo a process known as "choke-back", i.e., those contour lines are reduced in thickness until eliminated. Finally another negative is made from the positive depicting only the desired slope band.
The entire process is repeated for each slope band required. In optional additional steps, the original contour lines are superimposed on the negative, along with planographic details, such as roads, houses and the like.
Perhaps the most critical step in the process of slope mapping, as described above, is the step of spreading contour lines to form the desired slope band. In order to prepare a slope map depicting low percentage slope regions, e.g., in the range of 5 to 10%, the contour lines must be spread (thickened) by a substantial amount (up to one-half inch). In the preparation of high percentage slope regions, thickening of the contour lines is less, but must be extremely accurate to prevent unwanted overlap. In both cases, spreading of the contour lines must be uniform and undistorted in order to present an accurate representation of the slope band with a minimum of touching up required.
Although generally somewhat satisfactory for limited line weight manipulating, prior art apparatus of which we are aware are unsuitable for wide line spreading. For example, in U.S. Pat. No. 3,204,544 to Shannon, line spreading is effected by employing a controlled, oscillating, flat table top for supporting photographic film, wherein the movement of the table defines an orbit having a predetermined diameter. Shannon's system is useful for relatively small spreading; for large spreading, however, that system creates anomalies or "frills", that is, spurious edges in the thickened contour lines. The reason for the anomalies and spurious edges is that the table oscillates with a fixed diameter. Painting of the thickened line image onto the photosensitive film is not complete; gaps are left in some of the slope bands and there is loss of detail at the slope edges.
Furthermore, in Shannon, the diameter of the orbit is set by manually adjusting spring tension on a table bearing. Inaccuracies resulting from that type of mechanism would be unsuitable for making slope maps representative of high percentage slope regions, because, as aforementioned, adjacent contour lines may be separated by only about one mil.
In U.S. patent application Ser. No. 610,810 filed Sept. 5, 1975, now U.S. Pat. No. 4,029,410, issued June 14, 1977, assigned to the Assignee of the present invention, a lens orbiting attachment is provided between a cartographic camera and lens thereof. During the exposure, the lens is orbited about the optical axis of the camera in a plane parallel to the image. The diameter of the orbit is continuously stepped between zero, wherein the lens is concentric with the optical axis of the camera, and a predetermined maximum diameter that depends on the amount of line spreading required. That system is generally satisfactory, particularly for depicting low-slope regions, because the lens disposed between the negative or positive and film provides image magnification. In very steep-slope regions, however, and particularly small scale contour maps wherein contour lines may be separated by only one mil, any inaccuracies in the orbiting mechanism, magnified by the lens, makes precise line spreading difficult to obtain.